1. Field of the Invention
The present invention relates to reflectometry. More particularly, the present invention relates to a reflectometer system and method for obtaining thickness information by measuring phase shift in reflected split frequency signals via heterodyne interferometry. Furthermore, the present invention relates to a method and system for using the heterodyned signals from a heterodyne reflectometer for measuring the thicknesses of thin and ultra thin films formed over substrates. Still more particularly, the present invention relates to a heterodyne reflectometer for in situ monitoring of film thickness. Additionally, the present invention relates to grating interferometry and measuring phase shift in heterodyne signals resulting from reflection in a film and from diffraction in a grating. Even still more particularly, the present invention relates to a combination heterodyne reflectometer and grating interferometer for obtaining thickness information for a film from measuring phase shift in reflected heterodyned signals and obtaining a refractive index for the film from measuring grating induced phase shift in diffracted heterodyned grating signals, and dynamically updating the refractive index in a thickness calculation.
2. Description of Related Art
Semiconductors such as chips, microchips, or integrated circuits (ICs) are composed of a myriad of tiny transistors, aluminum or copper lines and electric switches, which manipulate the flow of electrical current. Semiconductor wafers are transformed into ICs by performing various processes on the wafer substrate and/or subsequently formed layers above the substrate; these include imaging, deposition and etching. A commonly held axiom within the semiconductor industry states that the density of transistors in an integrated circuit is expected to double every eighteen months. Thus, the implementation of new techniques for fabricating ever-smaller semiconductor structures is necessary to meet this goal. Due to the increasing demand for ultra precise tolerances in chip fabrication, the physical characteristics of the subsequent layers must be very carefully controlled during processing to achieve satisfactory results for most applications. One method for monitoring the depth and/or thickness of a layer or stack of layers is interferometry. Broadly defined, interferometry relates to the measurement of the interaction of waves, in this case, optical waves.
An interferometer works on the principle that two coherent waves that coincide with the same phase will enhance each other while two waves that have opposite phases will cancel each other out.
One prior art monitoring system utilizes interferometry for measuring variations in surface profiles, from which feature height information can be inferred. Hongzhi Zhao, et al., in “A Practical Heterodyne Surface Interferometer with Automatic Focusing,” SPIE Proceedings, Vol. 4231, 2000, p. 301, which is incorporated herein by reference in its entirety, discloses an interferometer for detecting a phase difference between reference heterodyne signal, and a measurement signal. Height information related to the sharp illumination point on the surface can be inferred from the measurement. Although the reference and measurement signals are generated by beams that are propagated over different paths, this is a common path interferometer. This approach is sometimes referred to as the common-axis approach or the normal-axis approach because the incident and reflected beams occupy a common path or axis to a target location, which is normal to the surface being examined.
One shortcoming of the common-path heterodyne interferometers known in the prior art is that the height information is calculated from an average height of the large illumination area of the reference signal. Thus, the accuracy of the results is adversely affected by surface roughness. Another limitation of the prior art common axis method is that it does not measure or calculate an actual thickness parameter for a film layer.
Other attempts in monitoring film thicknesses achieve heterodyning by frequency modulating the light source. U.S. Pat. No. 5,657,124 to Zhang, entitled “Method of Measuring the Thickness of a Transparent Material,” and U.S. Pat. No. 6,215,556 to Zhang, et al., entitled “Process and Device for Measuring the Thickness of a Transparent Material Using a Modulated Frequency Light Source,” disclose such devices, and are incorporated herein by reference in their entireties. With regard to these devices, a polarized light beam having a modulated frequency is directed to the target surface and heterodyne interference signals are detected from two rays, one reflected off the top surface of a target and a second from a bottom surface of a target. A thickness is determined from the number of beats per modulation period by comparing the heterodyned interference signals with the linearly modulated intensity of the light source. The principle drawback of these types of devices is that since the heterodyning is achieved by frequency modulating the source and thinnest film measurable is limited by its bandwidth.
Other heterodyne interferometers obtained a heterodyned signal from two separate beams, a first beam at a first frequency and polarization and a second beam at a second frequency and polarization. U.S. Pat. No. 6,172,752 to Haruna, et al., entitled “Method and Apparatus for Simultaneously Interferometrically Measuring Optical Characteristics in a Noncontact Manner,” and U.S. Pat. No. 6,261,152 to Ayer, entitled “Heterodyne Thickness Monitoring System,” which are incorporated herein by reference in their entireties, disclose this type of interferometer.
FIG. 1 is a diagram of a heterodyne thickness monitoring apparatus in which a pair of split frequency, orthogonally polarized beams are propagated in separate optical paths prior to being mixed and heterodyned, as is generally known in the prior art for use with a Chemical Mechanical Polishing (CMP) apparatus. Accordingly, heterodyne thickness monitoring system 100 generally comprises a CMP apparatus, a wafer 110 and a measurement optical assembly. Wafer 110 includes substrate 112 and film 114.
The measurement optical assembly generally comprises various components for detecting and measuring a Doppler shift in the optical frequency of the reflected beam, including laser source 140, beam splitter (BS) 144, polarization beam splitter (PBS) 146, beam quarter-wave plate 148, beam reflector 152, beam quarter-wave plate 150, mixing polarizer 143, photodetector 147, mixing polarizer 145, photodetector 149, and signal-processing assembly 140 electrically connected to the outputs of photodetectors 147 and 149.
In operation, laser diode 140 emits a beam having first linear polarized light component 102 at a first wavelength and second linear polarized component 103 at a second wavelength, but orthogonally polarized to the first polarization component. The first and second polarization components 102 and 103 propagate collinearly to BS 144 where a portion of both components are reflected to mixing polarizer 145 as beams 114 and 115 and then to detector 149 as beams 116 and 117, where signal I2 is produced.
The transmitted portions of polarization components 102 and 103 propagate to PBS 146 as beams 104 and 105. At PBS 146 component 104 follows a first transmission path as beam 120 and passes through reference quarter-wave plate 148 to reflector 152 and is reflected back through quarter-wave plate 148 as beam 122 (orthogonally polarized to beam 120), where it reflects at PBS 146 to mixing polarizer 143 and on to detector 147 as beam 124.
The second polarization component, from component 105, follows a separate transmission path, from the first path, as beam 120 and is orthogonally oriented to first polarization component 104 and, therefore, reflects off PBS 146, passes through quarter-wave plate 150 as beam 109 and propagates to optically transparent rotatable carrier 115. Beam 109 experiences partial reflection at the back surface rotatable carrier 115, the interface between substrate 112 and the top surface of film 114, thereby producing partially reflected beams 111S, 111T and 111B, respectively. Each of reflected beams 109S, 109T and 109B propagate back through quarter-wave plate 150, are transmitted through PBS 146 as beams 113S, 113T and 113B and propagate collinearly with beam 122 to mixing polarizer 145 as beams 124, 115S, 115T and 115B and then detected at photodetector 147 as signal I2. Importantly, I2 is produced from both beam 107, which oscillates at one optical frequency and interacts the film, and beam 120, which oscillates at another optical frequency and that propagates in a second optical path that does not interact with the film. Signals I1 and I2 are compared for finding a thickness measurement.
When the measurement beam undergoes an optical path length change, the beat signal will experience corresponding phase shift as shown in the simulated result depicted in the diagram of FIG. 2. There, the phase of beat signal I2 (plot 103) is depicted as being shifted by Δφ from beat signal I1 (plot 105) due to the change in the optical path length of partially reflected beam 111T from the top surface of film 114, when the surface is eroded by polishing.
As can be seen, in the measurement path beam 111B is transmitted through the wafer and is reflected from the front wafer surface. As the optical beam path through the wafer is shortened, the reflected optical frequency of beam 111B undergoes a Doppler shift. Thus, one optical frequency (beams 111S, 111B and 111T) interacts with the target while the second optical frequency (beam 122) does not. However, separating the reference beam and measurement beam in such a manner has the disadvantage of degrading the S/N ratio of the heterodyne interferometer and reducing measurement sensitivity.
Generally, the resolution of heterodyne interferometers known in the prior art is limited to approximately 6 Å, thus prior art heterodyne interferometers lack the resolution necessary for accurately measuring thin films or for monitoring small changes in thickness during processing.